Is there some halacha about bread not allowed
to be dairy? I was reading an advertisement in a store in Brooklyn
and it was written on the paper something about bread not being
allowed to be dairy. Is this a halacha? I have never heard of
this before, but I looked around and all the bread I saw was "pareve."
Maybe this halacha just never came my way.

Dear Name@Withheld,

Yes, it's prohibited
to bake bread either with milk or with meat. That is, bread must
be pareve. This is stated in Shulchan Aruch as follows:
"It is forbidden to knead dough for bread with milk, in
case a person should (accidentally) eat it with meat...It is similarly
forbidden to bake bread in the oven with meat fat "

But if you mark the bread with some identifying
symbol before baking, or if you bake it in an identifying
shape so that it's clear that this bread is not pareve,
then it is permitted to make it dairy or meaty. Since it's marked,
you won't mistake it as pareve. (Marking it after baking
doesn't help, and such bread may not be eaten.)

For example, I have heard that the Badatz Eida
Charedit kashrut supervisory board in Jerusalem requires all
dairy pastries to be either triangle or moon-shaped for this reason.